This invention relates to a radar device intended to be installed on a vehicle such as an automobile.
In order to improve ease and safety in the operation of an automobile, functions for monitoring and cruising after a front-running vehicle are sometimes provided. For realizing such functions, a radar device having a sensor such as a laser radar or a millimeter-wave radar may be installed on a vehicle (xe2x80x9cthe own vehiclexe2x80x9d of the user) for obtaining at least position data on a target object of detection either in front or behind such as a front-running vehicle or a guard rail. For such a vehicle-mounted radar device, it is desirable to adjust the direction of the light to be emitted (or the direction of the center point of the area to be scanned and hereinafter sometimes referred to as the optical axis) in order not to miss the front-running vehicle even when it is running on a curve. Such a technology has been described, for example, in Japanese Patent Publication 9-218265.
This prior art technology is for the case of a flat road with no slopes and with curves with nearly constant curvature such as large highways. Thus, the probability of losing track of a front-running vehicle may be kept low with such a technology if the road is flat and curves always at a constant rate but it is likely to miss the front-running vehicle where there are many slopes and the radius of curvature of the road changes frequently as inside a big city even if the optical axis of the radar device is adjusted according to this prior art technology. This is because the direction of the optical axis is determined by this technology on the basis of the curvature of the road and the lateral positions of the own and front-running vehicles. Since the optical axis could not be corrected in the vertical direction and since the optimum vertical direction of the optical axis could not be determined, the vertical direction of the optical axis remained the same. Thus, if the road is sloped, the front-running vehicle may move out of the detectable area either upward or downward and may cease to be detectable.
Moreover, since this prior art technology would treat S-shaped curves of roads as having a constant radius of curvature in determining the optimum vertical direction of the optical axis, a large error would result in the horizontal direction whenever there are fine changes in the curvature of the road and the front-running vehicle is likely to move sideways out of the target area of detection.
It is therefore an object of this invention to provide a radar device mounted to a vehicle which is capable of appropriately correcting the direction of the optical axis of its sensor and does not easily lose sight of a front-running vehicle even in the presence of slopes and S-shaped curves.
A radar device embodying this invention is a vehicle-mounted device and may be characterized as comprising an image taking means for obtaining an image including a road surface in front of or behind the vehicle (xe2x80x9cown vehiclexe2x80x9d) to which it is mounted, a sensor having a center axis for obtaining at least positional information on a target object of detection (such as another vehicle) in front of or behind the own vehicle and a control means for correcting the direction of the center axis of the sensor based on the image obtained by the image taking means. The control means may be characterized as including a line segment detecting means for detecting a line segment along a lane in which the own vehicle is traveling, a vector detecting means for detecting a vector indicative of the direction on a road-fixed coordinate system of the line segment obtained by the line segment detecting means, and a correcting means for controlling correction of horizontal and vertical directions of the center axis of the sensor so as to coincide with the direction of the vector detected by the vector detecting means.
In the above, the target object of detection is basically intended to be another vehicle on the road either in front of or behind the xe2x80x9cown vehiclexe2x80x9d to which the subject device is mounted, and xe2x80x9canother vehiclexe2x80x9d in this context is intended to include vehicles with other than four wheels such as motorbikes but the target object of detection may also include structures other than vehicles such as guard rails. The sensor is, for example, an instrument such as a laser or millimeter-wave radar adapted to irradiate a detection area with electromagnetic or sonic waves and to detect a target object of detection on the basis of reflected waves. This sensor need not necessarily be of a scan type and may be of a non-scan type but must be provided with directionality, having a specified detection area. Thus, the center axis of the sensor means the axis directed to the center of the detection area, not the physical center axis of the structure of the sensor. For this reason, this center axis is also hereinafter referred to as the optical axis, or the optical axis of the sensor. In the case of a sensor of the scan type, this is the center axis of the area to be scanned, or the center axis of the range to be scanned.
In the above, the expression xe2x80x9cline segment along a lane in which the own vehicle is travelingxe2x80x9d means a segment of a line (or an edge) which forms the image of a mark on the road on one side or both sides of the lane on which the own vehicle is traveling (such as white or yellow single, double or broken lines), a guard rail, a central road dividing zone, a protective wall and a boundary with a side walk or an optical flow obtained from the time-change of their images.
Detection of such a line segment (from an image) may be effected reasonably dependably by a so-called edge detection method if the (bright-dark) gradation is clear as in the case of a white line. In order to improve the dependability of detection by eliminating the image components corresponding to stains on the road and roadside structures or to lower the detection threshold such that even an edge with unclear gradation (such as the edge of a road-center divider) can also be reliably detected, it is preferable to preliminary limit the area of detection in terms of conditions, say, on the angular positions of the line segments to be detected. Such a detection area may be limited, as shown in FIG. 5C, to the angular range of 30xc2x0-60xc2x0 from the left-hand bottom corner of the image frame and that of 120xc2x0-150xc2x0 from the right-hand bottom corner (both measured in the counter-clockwise direction from the direction to the right as the likely areas for the lines to be detected to appear on the screen. It is easy and also natural to set the image taking means (the camera) at a position and in a direction such that the boundary lines of the lane in which the own vehicle is traveling (such as white lines and road-center divider) will fall inside such areas. By excluding the remainder of the areas on the image for the purpose of detection, the desired line segments alone can be detected more easily and dependably.
By using a radar device according to this invention, the center axis of the sensor is corrected to the direction on the road-fixed coordinate system of the line segments along the lane in which the own vehicle is traveling on the image of the front or back of the own vehicle. Thus, it is hardly likely to lose sight of a target object of detection (especially a vehicle in front or behind) even on a road with constantly changing radius of curvature or having many slopes. If line segments are detected from white lines on the road at a specified distance in front (corresponding to a specified position on the image in the vertical direction) and the center axis of the sensor is corrected to the direction of the line segment on the road-fixed coordinate system, for example, the center axis of the sensor can be corrected horizontally and vertically corresponding to the three-dimensional direction of the road in front of or behind the own vehicle, independent of the curvature or slope of the road in the vicinity where the own vehicle is currently traveling. According to this invention, since the direction of the center axis of the sensor moves upward and downward according to the slope of the road in front or behind, in particular, the vertical correction which has not been done in prior art technologies is coming to be possible and it becomes rare to lose sight of a vehicle in front even on a road with slopes.
In order to still better adapt to road conditions with S-shaped curves and slopes, it is preferable to detect a plurality of line segments and to select a vector from the directions of the individual line segments such that the road in front or behind can be clearly surveyed.
According to a preferable embodiment of the invention, the line segment detecting means serves to detect at least a pair of line segments on both sides of a lane on which the own vehicle is traveling and the vector detecting means detects the vector by determining a vanishing point on the image as intersection of extensions of this pair of line segments and detecting the vector from this vanishing point. This embodiment is preferable because the image taking means may be realized with a simple construction with only one camera.
For obtaining the vector from a vanishing point as explained above, it is preferable to include a partitioning means for partitioning the image obtained by the image taking means into a plurality of zones and that the line segment detecting means serves to detect pairs of line segments by detecting one of the pairs of line segments in each of these zones, and that the vector detecting means detects the vector by determining a plurality of vanishing points from these pairs of line segments, detecting triangular areas each having a top corner at corresponding one of the vanishing points and bottom corners at points on the most closely positioned one of the pairs of line segments, calculating shared areas between the triangular areas and the area of the lane on the image, selecting one of the triangular areas having the largest of the shared areas, and detecting the vector from the vanishing point corresponding to the selected one of the triangular areas. In the above, the area of the lane may be obtained as an area surrounded by the plurality of line segments.
According to an alternative embodiment of the invention, the line segment detecting means serves to detect a bottom pair of line segments in the lowest one of the horizontally elongated zones along the bottom edge of the image and the vector detecting means detects the vector by determining the vanishing point from the bottom pair of line segments, selecting a plurality of points scattered horizontally and vertically on the image within a specified area centered around the determined vanishing point, detecting triangular areas each having a top corner at corresponding one of the selected points and bottom corners at points on the bottom pair of line segments in the lowest zone, calculating shared areas between these triangular areas and the area of the lane on the image, selecting one of the triangular areas having the largest of the shared areas, and detecting the vector from the selected point corresponding to the selected one of the triangular areas.
With a device according to either of the above two embodiments, the center axis of the sensor is corrected from the coordinates of a plurality of vanishing points or points scattered around the one vanishing point such that a target object of detection such as a vehicle in front or behind cannot be lost sight of even on a road full of S-shaped curves and/or slopes.
The latter embodiment is particularly advantageous in that only one vanishing point needs to be calculated. In the former embodiment, it is preferable that the vector detecting means serves to determine excludable zones where a target object of detection overlaps or is considered likely to overlap either of the line segments on the image on the basis of position data on the target object of detection obtained from the sensor and to exclude the excludable zones thus determined when determining the vanishing points.
The direction of the center axis of the sensor may be effected periodically while the own vehicle is running or whenever its ignition switch is turned on but the process efficiency can be improved if it is arranged to be carried out only when xe2x80x9cnecessaryxe2x80x9d or only when one of predetermined conditions becomes satisfied. Such conditions may include that the object of detection is detected to have changed horizontal position on the image, that the own vehicle is detected to have changed its speed, that the own vehicle is detected to have become tilted with respect to the horizontal plane, and that the own vehicle is detected to have changed lanes.